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Stable Isotopic Composition of Atmospheric
Carbon Dioxide (13C and 18O) from the 
NOAA ESRL Carbon Cycle Cooperative Global Air
Sampling Network, 1990-2014

Version: 2015-10-26
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CONTENTS

1.       Data source and contacts
2.       Use of data
2.1      Citation
3.       Reciprocity 
4.       Warnings
5.       Update notes
6.       Introduction
7.       DATA - General Comments
7.1      DATA - Sampling Locations
7.2      DATA - File Name Description
7.3      DATA - Event with single parameter
7.4      DATA - Event with multiple parameters
7.5      DATA - QC Flags
7.6      DATA - Monthly Averages
8.       Data retrieval
9.       References

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1. DATA SOURCE AND CONTACTS

University of Colorado
Institute of Arctic and Alpine Research (INSTAAR)

James White, Bruce Vaughn, and Sylvia Michel			
Institute of Arctic and Alpine Research
Campus Box 450
University of Colorado, Boulder, CO 80309-0450 USA

telephone: (303) 735-5850
facsimile: (303) 492-6388
e-mail: sylvia.englund@colorado.edu
SIL web site: http://instaar.colorado.edu/sil

Jim White, CU-INSTAAR 
Sylvia Michel, CU-INSTAAR 
Bruce Vaughn, CU-INSTAAR

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2. USE OF DATA

These data are made freely available to the public and the
scientific community in the belief that their wide dissemination
will lead to greater understanding and new scientific insights.
The availability of these data does not constitute publication
of the data.  NOAA relies on the ethics and integrity of the user to
insure that ESRL receives fair credit for their work.  If the data 
are obtained for potential use in a publication or presentation, 
ESRL should be informed at the outset of the nature of this work.  
If the ESRL data are essential to the work, or if an important 
result or conclusion depends on the ESRL data, co-authorship
may be appropriate.  This should be discussed at an early stage in
the work.  Manuscripts using the ESRL data should be sent to ESRL
for review before they are submitted for publication so we can
insure that the quality and limitations of the data are accurately
represented.

2.1 CITATION

Please reference these data as 

   White, J.W.C., B.H. Vaughn, and S.E. Michel (2015), University of Colorado, 
   Institute of Arctic and Alpine Research (INSTAAR), Stable 
   Isotopic Composition of Atmospheric Carbon Dioxide (13C 
   and 18O) from the NOAA ESRL Carbon Cycle Cooperative Global 
   Air Sampling Network, 1990-2014, Version: 2015-10-26, 
   Path: ftp://aftp.cmdl.noaa.gov/data/trace_gases/co2c13/flask/.

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3. RECIPROCITY

Use of these data implies an agreement to reciprocate.
Laboratories making similar measurements agree to make their
own data available to the general public and to the scientific
community in an equally complete and easily accessible form.
Modelers are encouraged to make available to the community,
upon request, their own tools used in the interpretation
of the ESRL data, namely well documented model code, transport
fields, and additional information necessary for other
scientists to repeat the work and to run modified versions.
Model availability includes collaborative support for new
users of the models.

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4. WARNINGS

Every effort is made to produce the most accurate and precise
measurements possible.  However, we reserve the right to make
corrections to the data based on recalibration of standard gases
or for other reasons deemed scientifically justified.

We are not responsible for results and conclusions based on use
of these data without regard to this warning.

The following important information is CRITICAL for the 
successful use of this data!

Every effort is made to produce the most accurate and precise
measurements possible.  However, we reserve the right to make
corrections to the data based on recalibration of standard gases
or for other reasons deemed scientifically justified. We are not 
responsible for results and conclusions based on use
of these data without regard to this warning.

**********************
Attention must be paid to the flagging scheme, because we put 
ALL of our data (warts and all) into this data base. It is only 
through effective use of the flagging field that meaningful data 
can be identified.  Please see the section that explains flagging.
***********************

Special consideration for using CO2O18 data:

Users of O18 data should be aware that many flasks in the 
network have been sampled without drying the air. This can 
result in isotopic exchange between CO2 and H2O, which can 
significantly deplete d18O values (see Gemery et al, 1996).  
NOAA is working to dry ALL air at wet sites prior to sampling,
but a few 'wet' sites remain. As of May 2009, we have applied 
a new flagging scheme that makes more of the data from potentially
wet sites available for use. SEE MORE DETAILS IN "COMMENTS" 
SECTION BELOW.

Users of the O18 data are encouraged to please contact 
James White (James.White@colorado.edu) prior to using 
the O18 data for a current update.

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5. UPDATE NOTES

+++++++++++++++++++++++++++++++
Lab-wide notes:

2015-08-03

The 3-letter site identification code for Ushuaia, Argentina (TDF) was 
changed to USH to be consistent with the WMO GAWSIS. 

2011-10-07

We have introduced the term "measurement group", which identifies
the group within NOAA and INSTAAR that made the actual measurement.
We can now have multiple groups measuring some of the same trace 
gas species in our discrete samples.  

Measurement groups within NOAA and INSTAAR are 

  ccgg:  NOAA Carbon Cycle Greenhouse Gases (CCGG)
  hats:  NOAA Halocarbons and other Atmospheric Trace Species (HATS)
  arl:   INSTAAR Atmospheric Research Laboratory (ARL)
  sil:   INSTAAR Stable Isotope Laboratory (SIL)
  curl:  INSTAAR Laboratory for Radiocarbon Preparation and Research (CURL)

We have also changed the file naming convention (see section "DATA - FILE 
NAME DESCRIPTION").

+++++++++++++++++++++++++++++++
Project-specific notes:

2013-08-27

Coordinates of some of the sample locations have changed.
These changes improve the specified location based on new
information.  Changes tend to be minor and do not necessarily
reflect a change in the actual sampling location.

2011-10-01

The data file format has been modified to include the measurement group
and, additionally, the sample collection and analysis times now include
second information (e.g., 2011 03 15 23 06 12).  See section 7.3 for
details.

2010-10-01

The format of the NOAA ESRL data records has been changed to include
an estimate of the uncertainty associated with each measurement.  The
determination of the estimate is trace gas specific and described in
section 6 (INTRODUCTION).

+++++++++++++++++++++++++++++++
Parameter-specific notes:

*************************************
January 2013 - Second position flags moved to first and third positions:

Due to confusion resulting from our second position flags, they were moved
to either first or third position. All flags are applied automatically by code
except for '!' hand flags. Please see flagging section for more information.

November 2010 - New second position flags:

In order to improve the quality of the data, we have implemented 
additional flags in the second position. See more in the flagging 
section.

October 2009 Update - Correction for 17O:

All data are now processed with a revised correction for the 
influence of 17O (since 12C17O16O is an isobaric isotopologue of 13C16O2). 
Prior to this reprocessing (August 2009), the data were calculated 
based on the Craig (1957) measurements of the ratios of 17O and 13C 
in PDB carbonate. Craig also assumed a fractionation factor (lambda) 
between 17O and 18O of 0.5. However, work by Assonov and 
Brenninkmeijer (2003) suggests a more appropriate value of lamda is 
0.528, due to non-mass-dependent fractionation in the global 
hydrological cycle. Furthermore, their calculations of the isotopic
ratios of PDB are slightly different than those ascribed by Craig 
(1957). The resultant algorithms for the 17O correction were 
difficult to apply; however, the equations defined in Brand, 
Assonov, and Coplen (2009) simplify this alogrithm, and are now 
applied to our dataset. This is in keeping with recommendations 
provided by the WMO/IAEA CO2 Experts meeting held in Jena, Germany 
in 2008.

June 2009 Update - New and improved flagging of compromised data:

Historically, flasks were sampled at sites with high humidity 
without being dried, during some or all seasons. This is problematic
because there can be isotopic exchange in the flasks between CO2
and H2O (See Gemery et al, 1996). As of May 2009, we have 
implemented a data filtering method which distinguishes between 
samples that are compromised by water vapor from those that can be 
trusted as valid measurements.  See more details in "Comments" 
section below.

November 2006 Update - d18O scale change: 

The CO2O18 measurement scale has been adjusted by +0.82 
per mil, relative to V-PDB.  This was done as a result of 
calibrations and comparisons with multiple reference materials 
from other labs.  See the Comments section for more details. 
*************************************

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6. INTRODUCTION

The files in this archive contain measurements of the stable 
isotopic composition (13C and 18O) of atmospheric carbon dioxide. 
The isotopic analyses were carried out at the Stable Isotope 
Laboratory, CU-INSTAAR, using samples of air provided by the NOAA 
CMDL Carbon Cycle Cooperative Global Air Sampling Network.  For a 
complete listing of flask sampling site names and locations, see 
Section 7.1. 

Since 1989, the Stable Isotope Laboratory at INSTAAR has been 
measuring the stable isotopic composition of CO2 from weekly flask 
samples of air obtained from the CMDL network.  Starting with a 
selection of samples from just six sites and two ships in 1990, the 
measurement effort has grown  to include all sites in the NOAA/ESRL 
program. The degree to which isotopic measurements made on 
atmospheric samples are useful is seriously constrained by the 
precision of the analysis method used. A small change of just 0.01 
per mil in delta 13C (henceforth written as d13C) could have a large
effect on models of partitioning of carbon between the oceans and 
the biosphere. Therefore, a high precision instrument and high
accuracy calibrations are desirable.    

In 1990 we began making measurements of flask samples with a VG 
SIRA Series II mass spectrometer.  This instrument and extraction 
system produced an overall reproducibility of 0.03 per mil for d13C
and 0.05 per mille for d18O (1 sigma of 9 standard cylinders within 
a run). (See more about the uncertainty of measurements in the 
comments section). In 1996 a Micromass Optima mass spectrometer was 
purchased and fitted with a custom manifold and extraction system, 
based on the methods that were proven on the SIRA instrument. This 
system yielded better precision: +/- 0.012 per mil d13C and +/- 0.03
1 per mil for d18O.  

In 2005 a GV Instruments (formerly Micromass and now Elementar)
Isoprime dual Inlet mass spectrometer was brought online for 
analysis of Progammable Flask Packages (PFPs) which obtain samples 
from aircraft, towers, and also remote surface sites. The extraction
system is conceptually identical to the previous two instruments 
(described further the Comments section). The reproducibility for 
the Isoprime  dual inlet system is generally +/- 0.01 per mil for 
d13C and +/- 0.03 for d18O.  This instrument can also analyze 
network flasks and reference cylinders.  

In 2006, we acquired another GV Instruments Isoprime dual Inlet 
mass spectrometer which has been be fitted with extraction systems 
to handle CO2 extracted from whole air, CO2 evolved from carbonates 
in phosphoric acid, and CO2 equilibrated with water. With these 
three sample preparation systems, we hope to better calibrate CO2 
in air to both d13C and d18O measurement scales. 

We continue to cooperate with the many different efforts to cross 
calibrate between stable isotope labs, including flask inter-
comparisons, round-robin high pressure cylinder exchanges, and 
measurement of the new Jena Air Reference Set (JRAS), produced from CO2 
evolved from carbonates mixed with CO2-free air by the Max Planck 
Institute for Biogeochemistry. If, as a result of these calibration
efforts it becomes necessary to adjust our scale, we will document
it here.  For more information about the Stable Isotope Lab or the 
instruments used for theses analyses, including a downloadable 
document on methods, see the SIL web site at:  
http://instaar.colorado.edu/sil.  

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7. DATA - GENERAL COMMENTS

Carbon and oxygen isotopes of carbon dioxide (d13C and d18O of CO2)

The methods to measure isotope ratios of carbon and oxygen in carbon 
dioxide described below have been developed at the University of 
Colorado/INSTAAR Stable Isotope Laboratory. For more information, 
please refer to our web site: http://instaar.colorado.edu/sil or 
contact our assistant laboratory manager, Sylvia Michel: 
sylvia.englund@colorado.edu.

Methods:

To measure the stable isotopes of carbon dioxide, the CO2 must first
be separated from bulk air. A vacuum pulls the air from the flask 
or PFP through a mass flow controller, a water trap held at -85 
degrees C, and a liquid nitrogen trap at -197 degrees C. This 
extraction lasts for 10 minutes, sampling approximately 400 cc of 
air. After the CO2 sample has been isolated in the liquid nitrogen 
trap, a heating wire warms the sample to -20 degrees C, releasing 
it to the sample bellows of the dual inlet mass spectrometer. While
the isotope ratio is measured by the mass spectrometer (which takes 
approximately ten minutes), extraction of the next sample is 
underway, increasing the throughput of the instrument.

Mass spectrometry works on the principle that an ion in a magnetic 
field bends at a radius proportional to its mass-to-charge ratio. 
When the CO2 extracted from our sample is ionized by electrons, the 
ion beam is accelerated through a flight tube, separated by an 
electromagnetic field into masses 44, 45, and 46, and detected by 
Faraday cups. The currents obtained by these cups determine the 
ratio of heavy to light isotopes of the sample.

The sample is iteratively measured against a reference gas of 
isotopically similar CO2 in the reference bellows. Ten ratios are 
obtained, with a standard error usually less than 0.005. The 
isotopic value of the gas in the reference bellows is not known; to 
calibrate to the PDB scale, the international standard for carbon
and oxygen isotope ratios in CO2, we extract air from a cylinder 
with known isotope values of CO2 (the "working reference"). This 
follows the "identical treatment" principle, which states that 
samples and references should be handled using identical procedures 
to avoid introuducing bias.

The isotope ratios are reported as delta values (written here as 
"d") where d13C = [(13C/12Csample/(13C/12Cstandard/)-1] x 1000.
Delta values are reported in units of per mil. 
 
Corrections:

Several corrections are applied to the raw data. To correct for the
fractionation of gas in the reference bellows, we apply a drift 
correction using the working reference, which is run at the 
beginning, middle, and end of the run. Because N2O also freezes at 
liquid nitrogen and is isobaric with CO2, we mathematically 
correct for it using the concentration of N2O obtained by NOAA 
measurements (Mook and Jongsma 1987). We also correct for the 
presence of 12C17O16O, an isotopologue of 13CO2, using the Brand,
Coplen, and Assonov correction (2009; see more in the updates 
section). Finally, we anchor the measurements to the PDB scale 
using the calibrated isotopic values of our working reference.

Calibrations:

PDB (Pee Dee Bellemnite) is a carbonate, and its link to CO2-in-air
standards differs among laboratories.  Ongoing intercomparison 
experiments are essential for assessing the comparability of isotope
measurements from one lab with another (Masarie et al., 2001; 
Allison et al., 2003; Ghosh, P., Patecki, M., Rothe, M. and Brand, 
W.A. 2005). Measurement accuracy based on results from 
intercomparison experiments is 0.03 per mil. 

Uncertainty:

Samples are run daily from a cylinder whose isotope value is known: 
this "trap" tank alerts us to any problems with the mass 
spectrometer or extraction system. We also use this cylinder to 
calculate uncertainty, which we define as the standard deviation of 
ten runs' worth of trap measurements. Usually four trap samples are 
run a day, of which all but the first are used (due to known 
irregularities caused by the tank regulator). For each run, the 
uncertainty is calculated from the trap data from that run, 
regardless of its flagging, and the previous nine runs of unflagged 
data. (For example, if the current run is flagged, that trap data 
will be used in the uncertainty calculation; however, the next day's
run will not use the flagged data in its uncertainty calculation. 
Presumably, whatever caused one run to be flagged will have been 
repaired.) As of November 2009, uncertanty is calculated with each 
run, and has been back-calculated to 2004. On the Optima (flask 
measurement system) uncertainty averages 0.014 permil for d13C and 
0.035 permil for d18O; for the Isoprime (PFP system), uncertainty 
averages 0.017 permil for d13C and 0.04 permil for d18O. 

Special considerations for d18O of CO2: 

The INSTAAR-NOAA scale was known to be +0.82 per mil different from 
other labs measuring the d18O of CO2 in air, due to the temperature 
of the carbonate reaction that established the original CO2-in-air 
reference (Masarie et al., 2001; Allison et al., 2003). The 
application of a +0.82 per mil scale adjustment has been made as of 
November 2006 based on the results of the following comparisons: the 
CLASSIC  cylinders; standards prepared by Hitoshi Mukai's NARCIS I 
and II;  CO2 in air samples from W. Brand; flask inter comparisons 
with CSIRO (Cape Grim); and IAEA standard materials. 

If air is sampled without drying, oxygen can exchange between carbon
dioxide and water vapor in the flasks, in some cases entirely 
masking the desired signal (Gemery et al, 1996). While pair 
agreement flagging identifies most of the problem flasks, some 
flasks still have good pair aggreement and are within the normal 
range of acceptable values, even though they are biased by the 
influence of water vapor (Evans, 2008). 

Comparisons between d18O of CO2 and specific humidity, calculated 
from the National Climatic Data Center (NCDC), show that at tropical
and semi-tropical NOAA/ESRL sites, d18O of CO2 of non-dried samples 
has a seasonal cycle that is strongly anti-correlated to
the specific humidity, while the d18O of dried samples display a 
seasonal cycle that occurs 1-2 months earlier than the specific 
humidity seasonal cycle.  The latter phasing is expected, given the 
seasonal phasing between climate over the ocean and land; the 
former is consistent with a small, but measurable isotope exchange 
in the flasks.  

Based on investigations by Evans et al (2008), a threshold of 
specific humidity, below which non-dried air flask samples can be 
considered unaltered, has been set at 12 g/kg, and this filter has 
been applied to the d18O of CO2 database.  For example, for sites 
where specific humidity exceeds 12 g/kg, each non-dried flask sample
has been matched to a NCDC measurement of specific humidity within 
a 4-hour window. If specific humidity for that sample exceeded 
12 g/kg, the SIL measurement for d18O is flagged with a 'W' (for 
wet) in the first position.   If specific humidity data was 
unavailable, the sample is also flagged. Unfortunately there are 
several sites where the specific humidity is always above 12 g/kg 
(including CHR, GMI, POCN00, POCN05, POCN10, POCN15, POCS05, 
POCS10, POCS15, RPB, SEY, and SMO); therefore all data sampled 
without drying are flagged.

This approach segregates suspect and reliable data, providing a 
much-improved record of d18O of CO2 over the past two decades.  

Sample collection:

In the case of samples acquired from ships at sea, the Pacific 
Ocean Cruise (POC, travelling between the US west coast and New 
Zealand or Australia) data have been merged and grouped into 
5 degree latitude bins.  For the South China Sea cruises (SCS) the =
data are grouped in 3 degree latitude bins.

Sampling frequencies are approximately weekly for the fixed sites
and average one sample every 3 weeks per latitude zone for POC and
about one sample every week per latitude for SCS.

The air samples are collected by two general methods:  flushing and
then pressurizing glass flasks with a pump, or opening a stopcock on
an evacuated glass flask.  During each sampling event, a pair of
flasks is filled.  For more information on sampling units, see
http://www.esrl.noaa.gov/gmd/ccgg/psu/index.html

The Pacific Ocean Cruise (POC, travelling between the US west coast
and New Zealand or Australia) data have been merged and grouped into 
5 degree latitude bins.  For the South China Sea cruises (SCS) the 
data are grouped in 3 degree latitude bins.

Sampling frequencies are approximately weekly for the fixed sites
and average one sample every 3 weeks per latitude zone for POC and
about one sample every week per latitude for SCS.

Historically, samples have been collected using two general methods:
flushing and then pressurizing glass flasks with a pump, or opening a
stopcock on an evacuated glass flask; since 28 April 2003, only the
former method is used.  During each sampling event, a pair of flasks 
is filled.

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7.1 DATA - SAMPLING LOCATIONS

For a summary of sampling locations, please visit

<A HREF='http://www.esrl.noaa.gov/gmd/dv/site/site_table.html#ccg_surface'>http://www.esrl.noaa.gov/gmd/dv/site/site_table.html</A>.

IMPORTANT NOTES: 
1.  Data for all species may not be available for all sites listed 
in the table.
2.  The exact location of a sampling site recorded in our database
may change or become better defined over time.  The latitude,
longitude, and altitude of a sample event is based on the best
information available at the time of sample collection.  Differences
in sample position associated with a particular site may be due
to the site moving or changes in technology that permit a more
accurate location determination.

To view near real-time data, manipulate and compare data, and create
custom graphs, please visit

<A HREF='http://www.esrl.noaa.gov/gmd/dv/iadv/'>http://www.esrl.noaa.gov/gmd/dv/iadv/</A>.

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7.2 DATA - FILE NAME DESCRIPTION

Encoded into each file name are the parameter (trace gas identifier); sampling 
site; sampling project; laboratory ID number; measurement group; and optional 
qualifiers that further define the file contents.

All file names use the following naming scheme:

         1      2         3               4                   5                     6           7
[parameter]_[site]_[project]_[lab ID number]_[measurement group]_[optional qualifiers].[file type]

1. [parameter]

   Identifies the measured parameter or trace gas species.

   (ex)
   co2      Carbon dioxide
   ch4      Methane
   co2c13   d13C (co2)
   merge    more than one parameter

2. [site]

   Identifies the sampling site code.

   (ex)
   brw
   pocn30
   car
   amt

3. [project]
   
   Identifies sampling platform and strategy.

   (ex)
   surface-flask
   surface-pfp
   surface-insitu
   aircraft-pfp
   aircraft-insitu
   tower-insitu

4. [lab ID number]

   A numeric field that identifies the measurement laboratory (1,2,3, ...).
   NOAA ESRL is lab number 1 (see http://www.esrl.noaa.gov/gmd/ccgg/obspack/labinfo.html).

5. [measurement group]

   Identifies the group within NOAA or INSTAAR that makes the actual measurement.
   See Section 5 (UPDATE NOTES) for details.

   (ex)
   ccgg
   hats
   arl
   sil

6. [optional qualifiers]

   Optional qualifier(s) may indicate data subsetting or averaging.
   Multiple qualifiers are delimited by an underscore (_).  A more detailed
   description of the file contents is included within each data file.

   (ex)
   event         All measurement results for all collected samples (discrete (flask) data only).
   hour_####     Computed hourly averages for the specified 4-digit year (quasi-continuous data only)
   HourlyData    Computed hourly averages for entire record (quasi-continuous data only)
   DailyData     Computed daily averages for entire record (quasi-continuous data only)
   day           
   MonthlyData   Computed monthly averages for entire record (quasi-continuous data only)
   month         

7. [file type]
   
   We now provide some NOAA Global Monitoring Division measurements
   in two unique file formats (netCDF and ASCII text). The Network
   Common Data Form (NetCDF) is a self-describing, machine-independent
   data format that supports creation, access, and sharing of array-oriented
   scientific data.  To learn more about netCDF and how to read netCDF files,
   please visit http://www.unidata.ucar.edu.

   The ASCII text file is derived directly from the netCDF file.  The 
   text file is also self-describing and can be viewed using any text 
   editor.  "Self-describing" means the file includes enough information 
   about the included data (called metadata) that no additional file is 
   required to understand the structure of the data and how to read and 
   use the data.

   (ex) 

   txt           ASCII text file
   nc            netCDF4 file

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7.3 DATA - EVENT WITH SINGLE PARAMETER

The event data files in ftp://aftp.cmdl.noaa.gov/data/trace_gases/co2c13/flask/surface/ 
use the following naming scheme (see Section 7.2):

     [parameter]_[site]_[project]_[lab ID number]_[measurement group]_[optional qualifiers].txt

(ex) CH4_pocn30_surface-flask_1_ccgg.txt contains CH4 ccgg measurement
     results for all surface flask samples collected on the Pacific 
     Ocean Cruise sampling platform and grouped at 30N +/- 2.5 degrees.

(ex) CO2_brw_surface-flask_1_ccgg.txt contains CO2 ccgg measurement 
     results for all surface flask samples collected at Barrow, Alaska.

The data files contain multiple lines of header information followed by one 
record for each atmospheric measurement of a single parameter or trace gas species.

Fields are defined as follows:

Field 1:    [SITE CODE] The three-character sampling location code (see above).

Field 2:    [YEAR] The sample collection date and time in UTC.
Field 3:    [MONTH]
Field 4:    [DAY]
Field 5:    [HOUR]
Field 6:    [MINUTE]
Field 7:    [SECOND]

Field 8:    [FLASK ID] The sample container ID.

Field 9:    [METHOD] A single-character code that identifies the sample 
             collection method.  The codes are:

             P - Sample collected using a portable, battery
                 powered pumping unit.  Two flasks are
                 connected in series, flushed with air, and then
                 pressurized to 1.2 - 1.5 times ambient pressure.

             D - Similar to P but the air passes through a
                 condenser cooled to about 5 deg C to partially
	              dry the sample.

             G - Similar to D but with a gold-plated condenser.

             T - Evacuated flask filled by opening an O-ring sealed       
                 stopcock.

             S - Flasks filled at NOAA ESRL observatories by sampling
                 air from the in situ CO2 measurement air intake system.

             N - Before 1981, flasks filled using a hand-held
                 aspirator bulb. After 1981, flasks filled using a
                 pump different from those used in method P, D, or G.

             F - Five liter evacuated flasks filled by opening a
                 ground glass, greased stopcock.

Field 10:   [TRACE GAS NAME] Gas identifier (e.g., co2, co2c13).

Field 11:   [MEASUREMENT GROUP] Identifies the group within NOAA and 
             INSTAAR making the actual measurement (e.g., ccgg, hats, arl).
             See Section 5 (UPDATE NOTES) for details.

Field 12:   [MEASURED VALUE] Dry air mole fraction or isotopic composition.  
             Missing values are denoted by -999.99[9].

Field 13:   [ESTIMATED UNCERTAINTY] Estimated uncertainty of the reported
             measurement value.  Missing values are denoted by -999.99[9].

Field 14:   [QC FLAG] A three-character field indicating the results of our 
             data rejection and selection process, described in section 7.5.

Field 15:   [INSTRUMENT] A 2-character code that identifies the instrument 
             used for the measurement.

Field 16:   [YEAR] The measurement date and time in LT.
Field 17:   [MONTH]
Field 18:   [DAY]
Field 19:   [HOUR]
Field 20:   [MINUTE]
Field 21:   [SECOND]

Field 22:   [LATITUDE] The latitude where the sample was collected, (negative (-)
             numbers indicate samples collected in the Southern Hemipshere).

Field 23:   [LONGITUDE] The longitude where the sample was collected, (negative (-)
             numbers indicate samples collected in the Western Hemisphere).

Field 24:   [ALTITUDE] The altitude of the sample inlet (masl).

Field 25:   [EVENT NUMBER] A long integer that uniquely identifies the sampling
             event.

Fields in each line are delimited by whitespace.

(ex)

   SUM 2009 08 21 13 53 00 2145-99 P ch4 ccgg 1857.010 1.100 ... H11 2010 05 12 16 57 00 72.5800 -38.4800 3238.00 296091

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7.4 DATA - EVENT WITH MULTIPLE PARAMETERS

On special request we can distribute a "merged" file, which
includes for each sampling event, measurement results for muliple 
parameters or trace gas species.  A merged file does not include all 
information found in a single parameter data file.  For example,
merged files exclude measurement uncertainty, analysis instrument 
ID and date and time for each parameter.  Thus, the single parameter
data file is our most comprehensive data archive. 

The format of a merged file is slightly different from single parameter event file.
A "merged" file will have the word "merge" in the parameter field of the file name.  
The file name does not inform on the number of parameters included in the file.

Merged data files use the following naming scheme (see Section 7.2):

     merge_[site]_[project]_[lab ID number]_[measurement group]_[optional qualifiers].txt

(ex) merge_pocn30_surface-flask_1_ccgg.txt contains ccgg measurement results for two or
     more parameters for all surface flask samples collected on the Pacific Ocean Cruise 
     sampling platform and grouped at 30N +/- 2.5 degrees.

(ex) merge_brw_surface-flask_1_ccgg.txt contains ccgg measurement results for two or more
     parameters for all surface flask samples collected at Barrow, Alaska.

The data files contain multiple lines of header information followed by one 
record for each atmospheric measurement of a single parameter or trace gas species.

Fields are defined as follows:

Field 1:    [SITE CODE] The three-character sampling location code (see above).

Field 2:    [YEAR] The sample collection date and time in UTC.
Field 3:    [MONTH]
Field 4:    [DAY]
Field 5:    [HOUR]
Field 6:    [MINUTE]
Field 7:    [SECOND]

Field 8:    [FLASK ID] The sample container ID.

Field 9:    [METHOD] A single-character code that identifies the sample 
             collection method.  The codes are:

             P - Sample collected using a portable, battery
                 powered pumping unit.  Two flasks are
                 connected in series, flushed with air, and then
                 pressurized to 1.2 - 1.5 times ambient pressure.

             D - Similar to P but the air passes through a
                 condenser cooled to about 5 deg C to partially
	              dry the sample.

             G - Similar to D but with a gold-plated condenser.

             T - Evacuated flask filled by opening an O-ring sealed       
                 stopcock.

             S - Flasks filled at NOAA ESRL observatories by sampling
                 air from the in situ CO2 measurement air intake system.

             N - Before 1981, flasks filled using a hand-held
                 aspirator bulb. After 1981, flasks filled using a
                 pump different from those used in method P, D, or G.

             F - Five liter evacuated flasks filled by opening a
                 ground glass, greased stopcock.

Field 10:   [LATITUDE] The latitude where the sample was collected, (negative (-)
             numbers indicate samples collected in the Southern Hemipshere).

Field 11:   [LONGITUDE] The longitude where the sample was collected, (negative (-)
             numbers indicate samples collected in the Western Hemisphere).

Field 12:   [ALTITUDE] The altitude of the sample inlet (masl).

Field 13:   [EVENT NUMBER] A long integer that uniquely identifies the sampling
             event.

There is a group of 4 fields for each parameter and measurement group included in the 
merge file.

Field ##+1: [TRACE GAS NAME] Gas identifier (e.g., co2, co2c13).

Field ##+2: [MEASUREMENT GROUP] Identifies the group within NOAA and 
             INSTAAR making the actual measurement (e.g., ccgg, hats, arl).
             See Section 5 (UPDATE NOTES) for details.

Field ##+3: [MEASURED VALUE] Dry air mole fraction or isotopic composition.  
             Missing values are denoted by -999.99[9].

Field ##+4: [QC FLAG] A three-character field indicating the results of our 
             data rejection and selection process, described in section 7.5.

Fields in each line are delimited by whitespace.

(ex)

   SGP 2002 04 02 21 15 00 551-91 D 36.6200 -97.4800 374.00 115605 co2 CCGG 378.870 -.. ch4 CCGG 1874.995 ... 
   co CCGG 163.800 ... h2 CCGG 496.350 ... n2o CCGG 319.380 ..P sf6 CCGG 5.130 ..P co2c13 SIL -8.477 +..
   co2o18 SIL -0.271 ... 

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7.5 QC FLAGS

NOAA ESRL uses a 3-column quality control flag where each column
is defined as follows:

column 1    REJECTION flag.  An alphanumeric other
            than a period (.) in the FIRST column indicates
            a sample with obvious problems during collection
            or analysis.  This measurement should not be interpreted.

column 2    SELECTION flag.  An alphanumeric other than a
            period (.) in the SECOND column indicates a sample
            that is likely valid but does not meet selection
            criteria determined by the goals of a particular
            investigation.

column 3    INFORMATION flag.  An alphanumeric other than a period (.) 
            in the THIRD column provides additional information 
            about the collection or analysis of the sample.

            WARNING: A "P" in the 3rd column of the QC flag indicates
            the measurement result is preliminary and has not yet been 
            carefully examined by the PI.  The "P" flag is removed once 
            the quality of the measurement has been determined.

Samples are collected in pairs, and in order for a value to be 
retained, the flask pair difference must meet the threshold tolerance
for the specific instrument being used.  For samples analyzed prior 
to 1996 pairs must agree to within 0.09 per mil for C13 and 0.15 per 
mil for O18. For post-1996 analyses, done on both Optima and Isoprime 
mass spectrometers, pairs must agree to within 0.06 per mil for C13 
and 0.12 per mil for O18.

Typically, pairs are flagged for pair rejection more frequently for 
d18O of CO2 than for d13C of CO2.  The reason for the lower success 
rate in O18 pair agreement is likely related to the isotopic exchange 
of water vapor and CO2 in the flask during storage and analysis. This 
is dramatically demonstrated in the particularly low pair agreements 
at high humidity, low latitude sites. See Section 7, Data-General 
Comments for more details on the O18 data set.


FLAG DEFINITIONS

*********************Reject flags (1st flag character)********************

Automatic flags - applied daily during data reduction by a processing program

   A   problems in analysis or data reduction. Typically, the A 
   is applied when the working references run at the beginning, 
   middle, or end of the run have a standard deviation higher 
   than 0.04 or 0.08 permil for d13C and d18O, respectively. For
   measurements prior to 1996, values differ by more than 0.075 
   and 0.14 permil, respectively. 
 
   +   bad pair (high member). Values differ by more than 0.06 
   or 0.12 permil for d13C and d18O, respectively. For 
   measurements prior to 1996, values differ by more than 0.09 
   and 0.15 permil, respectively)

   -   bad pair (low menber). See above.

   H  The trap tank run on the same day has an average d13C or d18O value 
   greater than 0.15 or 0.3 permil above its long term averages, 
   respectively (since 2005.)

   L  The trap tank run on the same day has an average d13C or d18O value 
   less than 0.15 or 0.3 permil above its long term averages, 
   respectively (since 2005.)

   .   good flask
   
Flags applied periodically by code

   C   flagged for CO2 mole fraction (by NOAA CO2 measurement lab)

   W   flask sampled 'wet' (applied to o18 data only). See 
   comments.
   
   N/n  problem due to sample collection (inherited from CO2 
   measurements) 

Hand flags - applied by hand or in software (REFLAG) to selected flasks

   !   hand flag

*******************Non-background flags (2nd flag character)****************

ground flags - applied by outlier-identification software (CCG_FILTER)

   X  Outlier by more than 3-sigma from a CCGVU curve

   x  Outlier by more than 3-sigma in CO2 concentration

   .  Good flask

Hand flags - applied by hand to selected flasks

   !   hand flag

*********************Retain flags (3rd flag character)***********************

Automatic flags - applied during data reduction (by the processing program)

   S - single flask (flask without a pair mate)

   o - no trap data available for comparisons
   
   
   H  The trap tank run on the same day has an average d13C or d18O 
   value greater than 0.045 or 0.09 permil above its long term
   averages, respectively (since 2005.)

   L  The trap tank run on the same day has an average d13C or d18O 
   value less than 0.045 or 0.09 permil above its long term
   averages, respectively (since 2005.)
   
   P  Data has poor precision in dual inlet analysis: greater 
   than 0.02 permil for d13C and 0.03 permil for d18O (since 2005).

   T  The trap tank run on the same day has high standard 
   deviation of its (typically) three measurements: above 0.08 
   permil and 0.16 permil for d13C and d18O respectively (since 2005).

Additional flags
 
   L   linked flask (0.5-liter flask analyzed together with its 
   mate)
   
   I   flask also analyzed by another lab (aliquot taken)
   
   i   same as "I" above, but displaced a previous flag in this 
   field.
   
   .   no comments 

-------------------------------------------------------------------
7.6 DATA - MONTHLY AVERAGES

The monthly data files in ftp://aftp.cmdl.noaa.gov/data/trace_gases/co2c13/flask/surface/ 
use the following naming scheme (see Section 7.2):

     [parameter]_[site]_[project]_[lab ID number]_[measurement group]_month.txt

(ex) CH4_pocn30_surface-flask_1_ccgg_month.txt contains CH4 ccgg monthly
     mean values for all surface flask samples collected on the Pacific
     Ocean Cruise sampling platform and grouped at 30N +/- 2.5 degrees.

(ex) CO2_brw_surface-flask_1_ccgg_month.txt contains CO2 ccgg monthly
     mean values for all surface flask samples collected at Barrow, Alaska.

Monthly means are produced for each site by first averaging all
valid measurement results in the event file with a unique sample
date and time.  Values are then extracted at weekly intervals from 
a smooth curve (Thoning et al., 1989) fitted to the averaged data 
and these weekly values are averaged for each month to give the 
monthly means recorded in the files.  Flagged data are excluded from the
curve fitting process.  Some sites are excluded from the monthly
mean directory because sparse data or a short record does not allow a
reasonable curve fit.  Also, if there are 3 or more consecutive months
without data, monthly means are not calculated for these months.

The data files contain multiple lines of header information 
followed by one line for each available month.

Fields are defined as follows:

Field 1:    [SITE CODE] The three-character sampling location code (see above).

Field 2:    [YEAR] The sample collection year and month.
Field 3:    [MONTH]

Field 4:    [MEAN VALUE] Computed monthly mean value

In these files a monthly mean value of -999.99 denotes months where there
are insufficient data to calculate a monthly mean.

-------------------------------------------------------------------
8. DATA RETRIEVAL

To transfer all files in a directory, it is more efficient to 
download the tar or zipped files.  

To transfer a tar file, use the following steps from the ftp prompt:

   1. ftp> binary                    ! set transfer mode to binary
   2. ftp> get filename.tar.gz       ! transfer the file
   3. ftp> bye                       ! leave ftp

   4. $ gunzip filename.tar.gz       ! unzip your local copy
   5. $ tar xvf filename.tar         ! unpack the file

To transfer a zipped file, use the following steps from the ftp prompt:

   1. ftp> binary                    ! set transfer mode to binary
   2. ftp> get filename.zip          ! transfer the file
   3. ftp> bye                       ! leave ftp

   4. $ unzip filename.zip           ! uncompress your local copy

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9. REFERENCES

For more publications see the Stable Isotope Lab web site at: 
http://instaar.colorado.edu/research/labs-groups/stable-isotope-laboratory/publications-detail/

Assonov, S.S., and Brenninkmeijer, C.A.M. 2003. On the 17O correction
for CO2 mass spectrometric isotopic analysis. Rapid Communications in
Mass Spectrometry 17(10): 1007-1016.
 
Battle. M., M.L. Bender, P.P. Tans, J.W.C. White, J.T. Ellis, 
T. Conway, and R.J. Francey. 2000. Global carbon sinks and their 
variability inferred from the atmospheric O2 and d13C. 
Science 287: 2467-2470.   

Brand, W.A., S.S. Assonov, and T.B. Coplen. 2009. Correction for 
the 17O interference in d13C measurements when analyzing CO2 with 
stable isotope mass spectrometry (IUPAC Technical Report). Journal 
of Pure and Applied Chemistry 82(8): 1719-1733.

Ciais, P., P.P. Tans, J.W.C. White, M. Trolier, R.J. Francey, 
J.A. Berry, D.R. Randall, R.J. Sellers, J.G. Collatz and 
D.S. Schimel. 1995. Partitioning of ocean and land uptake of CO2 
as inferred by d13C measurements  from the NOAA/CMDL global air 
sampling network, Journal of Geophysical Research 100: 5051-5070.  

Ciais, P., P.P. Tans, M. Trolier, J.W.C. White and R.J. Francey. 
1995. A  large northern hemisphere terrestrial CO2 sink indicated 
by the 13C/12C ratio  of atmospheric CO2. Science 269: 1098-1102.  

Ciais, P., A.S. Denning, P.P. Tans, J.A. Berry, D.A. Randall, 
G.J. Collatz, P.J. Sellers, J.W.C. White, M. Trolier, H.A.J. Meyer, 
R.J. Francey, P. Monfray, and M. Heimann. 1997. A three-dimensional 
synthesis study of d18O in atmospheric CO2. 1. Surface fluxes. 
Journal of Geophysical Research 102: 5857-5872.  

Ciais, P., P.P. Tans, A.S. Denning, R.J. Francey, M. Trolier, 
H.A.J. Meyer, J.W.C. White, J.A. Berry, D.A. Randall, G.J. Collatz, 
P.J. Sellers, P. Monfray, and M. Heimann. 1997. A three-dimensional 
synthesis study of d18O  in atmospheric CO2. 2. Simulations with the
TM2 transport model. Journal of Geophysical Research 102: 5873-5883.  

Conway, T.J., P.P. Tans, L.S. Waterman, K.W. Thoning, D.R. Kitzis, 
K.A. Masarie, and N. Zhang. 1994. Evidence for interannual variability
of the  carbon cycle from the NOAA/CMDL global air sampling network.
Journal of Geophysical Research 99: 22831- 22855.  

Cuntz, M., P. Ciais, G. Hoffmann, C.E. Allison, R.J. Francey, 
W. Knorr, P.P. Tans, J.W.C. White, I. Levin. 2003. A comprehensive
global three- dimensional model of delta O-18 in atmospheric  CO2: 
2. Mapping the atmospheric signal. Journal of Geophysical, Research-
Atmospheres 108(D17): article 4528. 

Evans, C.U. 2008. d18O of atmospheric carbon dioxide: Towards the 
development of an artifact free database from the NOAA/ESRL Carbon 
Cycle Cooperative Global Air Sampling Network. Masters Thesis, 
INSTAAR, University of Colorado, Boulder.  

Francey, R.J., P.P. Tans, C.E. Allison, I.G. Enting, J.W.C. White 
and M. Trolier. 1995. Changes in oceanic and terrestrial carbon 
uptake since 1982. Nature 373: 326-330.  

Gemery, P.A., M. Trolier, and J.W.C. White. 1996. Oxygen isotope 
exchange  between carbon dioxide and water following atmospheric 
sampling using glass  flasks. Journal of Geophysical Research -
Atmospheres 101: 14415-14420.  

Ghosh, P., M. Patecki, M. Rothe and W.A. Brand. 2005. Calcite-CO2 
mixed into CO2-free Air: A New CO2 in-Air Stable Isotope Reference 
Material for the VPDB Scale. Rapid Communications in Mass 
Spectrometry 19: 1097-1119.  

Miller, J.B., P.P. Tans, J.W.C. White, T.J. Conway, B.H. Vaughn.
2003. The atmospheric signal of terrestrial carbon isotopic 
discrimination and its implication for partitioning carbon fluxes.  
Tellus Series B-Chemical and Physical Meteorology 55(2): 197-206.

Mook, W.G. and J. Jongsma. 1987. Measurement of the N2O correction
for 13C/12C ratios of atmospheric CO2 by removal of N2O. Tellus 39B:
96-99.

Tans, P.P., T.J. Conway, and T. Nakazawa. 1989a. Latitudinal 
distribution of  the sources and sinks of atmospheric carbon dioxide
from surface observations and an atmospheric transport model.
Journal of Geophysical Research 94: 5151-5172.  

Tans, P.P, K.W. Thoning, W.P. Elliott, and T.J. Conway. 1989b.
Background atmospheric CO2 patterns from weekly flask samples at 
Barrow, Alaska: Optimal signal recovery and error estimates, in 
NOAA Tech. Memo. (ERL ARL- 173). Environmental Research 
Laboratories, Boulder, CO, 131 pp.  

Tans, P.P., I.Y. Fung, and T. Takahashi. 1990. Observational 
constraints on the global atmospheric CO2 budget. Science 247:
1431-1438.  

Thoning, K.W., P.P. Tans, and W.D. Komhyr. 1989. Atmospheric carbon 
dioxide  at Mauna Loa Observatory 2. Analysis of the NOAA GMCC data, 
1974-1985, Journal of Geophysical Research 94: 8549-8565.  

Trolier, M., J.W.C. White, P.P. Tans, K.A. Masarie and P.A. Gemery.
1996. Monitoring the isotopic composition of atmospheric CO2: 
measurements from the NOAA Global Air Sampling Network. Journal of
Geophysical Research 101: 25897-25916.   

Vaughn, B., Ferretti, D., Miller, J. & White, J. 2004: Stable isotope
measurements of atmospheric CO2 and CH4. Handbook of Stable Isotope 
Analytical Techniques, vol 1, ch.14, Elsiever, 1248 p. 

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